Processes for growing carbon nanotubes in the absence of catalysts

ABSTRACT

Processes for increasing the production rate of single-wall carbon nanotubes using a disordered carbon target are disclosed. The processes use a disordered carbon target and include vaporization of the target in the presence of a non-oxidizing gas. The single-wall nanotubes produced can be incorporated into electronic devices such as diodes and transistors.

FIELD OF THE INVENTION

The present invention relates to processes for single-wall carbonnanotube production. By using a disordered carbon target, the processescan provide increased production rates as compared to known processes.

BACKGROUND OF THE INVENTION

In the field of molecular nanoelectronics, few materials show as muchpromise as nanotubes, and in particular carbon nanotubes, which comprisehollow cylinders of graphite. Nanotubes can be incorporated intoelectronic devices such as diodes and transistors, depending on thenanotube's electrical characteristics. Nanotubes are unique for theirsize, shape, and physical properties. Structurally, a carbon-nanotuberesembles a hexagonal lattice of carbon rolled into a cylinder.

Besides exhibiting intriguing quantum behaviors at low temperature,carbon nanotubes exhibit the following important characteristics: ananotube can be either metallic or semiconductor depending on itschirality (i.e., conformational geometry). Metallic nanotubes can carryextremely large current densities. Semiconducting nanotubes can beelectrically switched on and off as field-effect transistors (FETs). Thetwo types may be covalently joined (sharing electrons). Thesecharacteristics point to nanotubes as excellent materials for makingnanometer-sized semiconductor circuits.

Nanotubes can be formed as single-wall carbon nanotubes (SWNTs) ormulti-wall carbon nanotubes (MWNTs). SWNTs can be produced, for example,by arc-discharge and laser ablation of a carbon target. Local growth oftubes on a surface can also be obtained by chemical vapor deposition(CVD). The growth of the nanotubes is made possible by the presence ofmetallic particles, such as Co, Fe, and/or Ni, acting as catalyst. Theresultant carbon nanotubes typically contain contaminants, e.g.,catalyst particles. For some potential nanotube applications the use ofclean nanotubes can be important, such as, for example, where nanotubesare incorporated as an active part of electric devices. The presence ofcontaminating atoms and particles can alter the electrical properties ofthe nanotubes. The metallic particles can be removed; however theprocess of cleaning or purifying the nanotubes can be complicated andcan alter the quality of the nanotubes.

U.S. patent application Ser. No. 2004/0035355 discloses a method forgrowing single-wall nanotubes comprising providing a silicon carbidesemiconductor wafer comprising a silicon face and a carbon face, andannealing the silicon carbide semiconductor wafer in a vacuum at atemperature of at least about 1,350° C., thereby inducing formation ofsingle-wall carbon nanotubes on the silicon face. The disclosed processis carried out at 10⁻⁹ Torr, which is not generally conducive to highmaterial production rates.

SWNTs have been identified as potential components of electronic devicesin varied applications. Therefore, a need exists for new and/or improvedprocesses for growing single-wall carbon nanotubes.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process comprising:

a) providing a target comprising disordered carbon and a metal catalyst,wherein the carbon target has a density of 0.01 to 2 gm/cm³;

b) vaporizing the target at a temperature from about 900° C. to about1500° C. and at a pressure from about 10⁻³ Torr to about 10⁺³ Torr inthe presence of a non-oxidizing gas; and

c) forming a product comprising at least one single-wall carbonnanotube.

In some preferred embodiments, the target comprises about 10 weightpercent or less of the metal catalyst.

Another aspect of the present invention is a single-wall carbon nanotubemade by a process comprising:

a) providing a target comprising disordered carbon and a metal catalyst,wherein the carbon target has a density of 0.01 to 2 gm/cm³;

b) vaporizing the target at a temperature from about 900° C. to about1500° C. and at a pressure from about 10⁻³ Torr to about 10⁺³ Torr inthe presence of a non-oxidizing gas; and

c) forming a product comprising at least one single-wall carbonnanotube.

These and other aspects of the present invention will be apparent tothose skilled in the art, in view of the following description and theappended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the production rate as a function of target densityaccording to an embodiment of present invention.

DETAILED DESCRIPTION OF THE INVENTION

All documents cited herein are expressly incorporated herein byreference in their entirety. Applicants also incorporate herein byreference the co-owned and concurrently filed application entitled“PROCESSES FOR GROWING CARBON NANOTUBES IN THE ABSENCE OF CATALYSTS”.(Attorney Docket # CL 2626).

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

The present invention provides processes for producing single-wallcarbon nanotubes (SWNT). In preferred embodiments, the processes provideincreased rates of production of single-wall carbon nanotubes. It hasbeen found that the use of a relatively lower density carbon target(sometimes referred to as “softer” carbon target, e.g., about 2.2 g/cm³or less) can provide increased rates of production of nanotubes ascompared with known methods such as conventional laser ablation ofgraphite, arc discharge, chemical vapor deposition, and high pressurecarbon monoxide techniques. As examples only, using a target having adensity of 1.6 g/cm³, it has been observed that the rate of productionof nanotubes was from about 0.1 to 0.5 grams per hour, and with adensity of 0.9 g/cm³, the rate observed was about 1.2-1.5 g/hour. It isnot intended that the invention be limited by these recited rates, asvariations in the process within the scope of the invention, which maybe made by one skilled in the art, can result in varied rates ofproduction of nanotubes.

In highly preferred embodiments, the processes include providing atarget that is a mixture of a metal catalyst and disordered carbon.Suitable metal catalysts include yttrium, iron; nickel and cobalt andcombinations thereof. By “disordered carbon” is meant a carbon materialhaving a density less than 2.2 g/cm³. Forming the target can beaccomplished, for example, by forming a mixture of catalyst, carbon anda graphite cement in a volatile solvent, allowing the solvent toevaporate, then compression molding the residual solid. The target thuscan have a density of 0.01 to 2 gm/cm³. The compression-molded articlecan be optionally heated, preferably in an inert atmosphere, tosubstantially remove traces of the solvent.

Vaporization of the target can be carried out by laser ablation or othersuitable methods, such as, for example, radio frequency inductionheating and sputtering. The vaporization can be carried out attemperatures between about 900° C. and 1500° C., preferably from about1000° C. to about 1300° C. Also, the vaporization can be carried out ata pressure of about 10⁻³ Torr to about 10⁺³ Torr, preferably from about300 Torr to 600 Torr. The vaporization is carried out in the presence ofa non-oxidizing gas, such as argon, neon, helium, nitrogen or mixturesthereof. It is generally desirable to grow the tubes at pressurespreferably 1 millitor or higher, and more preferably at 500 Torr orhigher. In some preferred embodiments, the pressure is about 1000 Torr.It is generally not desirable that the pressure be greater than about1000 Torr. Although a reduction in pressure below about 500 Torr has notbeen observed to undesirably affect the rate of growth of nanotubes,pressures of about 500 Torr or greater are often practical.

In one embodiment of this invention, the SWNT-containing product canserve as a target for one or more additional cycles of vaporization andSWNT-formation.

The process can further include an annealing step. The annealing can beperformed in an ultra-high vacuum (UHV) (e.g., at a pressure less thanabout 10⁻⁹ Torr), or at higher pressures, even above atmosphericpressure (760 Torr).

According to an embodiment of the present invention, vaporizing thedisordered carbon target can induce the growth of SWNTs. The vaporizingcan be performed under reduced pressure (e.g., at a pressure of about300-600 Torr) in non-oxidizing conditions and at temperatures betweenabout 900° C. and 1500° C. The process produces nanotubes, which arepreferably predominately SWNTs. The SWNTs can be very long and have agood crystalline quality. “Good crystalline quality” means substantiallyfree of observable defects. By further increasing the extent of carbondisorder, which is reflected in a decrease in the density, theproduction rate of SWNTs can be increased. All of the compositions andprocesses disclosed and claimed herein can be made and executed withoutundue experimentation in light of the present disclosure. While thecompositions and processes of this invention have been described interms of preferred embodiments, it will be apparent to those of skill inthe art that variations can be applied to the compositions and processesand in the steps or in the sequence of steps of the method describedherein without departing from the concept, spirit, and scope of theinvention. All such substitution and modifications apparent to thoseskilled in the art are deemed to be within the spirit, scope, andconcept of the invention as defined by the appended claims.

EXAMPLES

These Examples show the effects of the amount of disorder of the carbontarget on the nanotube production rate.

Example 1

Carbon black powder (66 grams (g), Alfa Aesar, Ward Hill, Mass.), nickelmetal catalyst powder (7.56 g, 2.2-3 μm stock # 10255, Alfa Aesar, WardHill, Mass.), cobalt metal catalyst powder (7.56 g, 1.6 μm stock #10455, Alfa Aesar, Ward Hill, Mass.), and Dylon® graphite cement (gradeC, 94.12 g, Dylan Industries, Inc., Cleveland, Ohio) were incorporatedinto a homogeneous mixture using 260 mL of methanol. The mixture wasallowed to dry and then was broken up. The mixture as made providestargets with a high level of disorder and a density of 0.93 g/cm³. Aportion of this mixture was placed into a stainless steel die (3.18 cmdiameter by 7.62 cm high). The die was placed in a press and heated to130° C. Force up to 40034 Newtons was then applied for 1 hour. The diewas allowed to cool before the target was removed. The targets were thenheated in an Ar atmosphere to 1150° C. The targets were then placed in anon-oxidizing atmosphere of Ar at 500 Torr, heated to 1100° C. andablated with two Nd:YAG lasers operating at 1064 nm and 30 Hz. Afelt-like material containing single-wall carbon nanotubes was formedand collected downstream in a cool zone. This felt-like material wasgenerated at a rate of 1.3 g/hr.

Example 2

The target preparation described in Example 1 was repeated using 187.2 gof Dylan® graphite cement, 7.56 g of nickel catalyst powder, 7.56 g ofcobalt metal catalyst powder, and 50 mL of methanol. The mixture as madeprovided targets with a density of 1.4 g/cm³ and less disorder than thetargets of Example 1. A portion of this mixture was placed into astainless steel die (3.18 cm diameter by 7.62 cm high). The die wasplaced in a press and heated to 130° C. Force of 40034 Newtons was thenapplied for 1 hour. The die was allowed to cool before the target wasremoved. The targets were then heated in an Ar atmosphere to 1150° C.The targets were then placed in an atmosphere of Ar at 500 Torr, heatedto 1100° C., and ablated with two Nd:YAG lasers operating at 1064 nm and30 Hz. A felt-like material containing nanotubes was formed andcollected downstream in a cool zone. The felt-like material wasgenerated at a rate of 0.9 g/hour. This production rate was lower thanthat obtained with the targets of 0.93 g/cm³ density of Example 1.

Example 3

The target preparation described in Example 1 was repeated using 94.12 gof Dylon® graphite cement, 7.56 g of nickel catalyst powder, 7.56 g ofcobalt metal catalyst powder, 66 g of graphite powder (grade UCP-1-M,Carbone of America, Ultra Carbon Division, Bay City, Mich.), and 130 mLof methanol. The mixture as made provides targets with a density of 1.6g/cm³ and even less disorder than the targets of Examples 1 and 2. Aportion of this mixture was placed into a stainless steel die (3.18 cmdiameter by 7.62 cm high). The die was placed in a press and heated to130° C. Force up to 40034 Newtons was then applied for 1 h. The die wasallowed to cool before the target was removed. The targets were thenheated in an Ar atmosphere to 1150° C. The targets were then placed inan atmosphere of Ar at 500 Torr, heated to 1100° C. and ablated with twoNd:YAG lasers operating at 1064 nm and 30 Hz. A felt-like materialcontaining nanotubes was formed and collected downstream in a cool zone.The felt-like material was generated at a rate of 0.4 g/hour. Thisproduction rate was lower than that achieved with the targets of 0.93g/cm³ or 1.4 g/cm³ density of Examples 1 and 2.

1-8. (canceled)
 9. A process for making a single wall carbon nanotubecomprising (a) providing a target having a density of 0.01 to 2 g/cm³comprising disordered carbon and a metal catalyst, wherein thedisordered carbon consists essentially of carbon black, and wherein thedisordered carbon has a density of less than 2.2 g/cm³; (b) laserablating the target at a temperature of between about 900° C. and 1500°C. and at a pressure of about 10⁻³ Torr to about 10⁺³ Torr in thepresence of a non-oxidizing gas; and (c) forming a product comprising atleast one single-wall carbon nanotube.
 10. The process of claim 9wherein the pressure is about 300 Torr to about 600 Torr.
 11. Theprocess of claim 9 wherein the temperature is about 1000° C. to about1300° C.
 12. The process of claim 9 wherein the non-oxidizing gas isselected from the group consisting of argon, neon, helium, nitrogen andmixtures thereof.
 13. A single-wall carbon nanotube produced by theprocess of claim
 9. 14. The process of claim 9 wherein the catalyticmetal comprises yttrium.
 15. The process of claim 9 further comprising astep of subjecting a product comprising at least one single-wall carbonnanotube to additional cycles of vaporization and single-wall carbonnanotube formation.
 16. The process of claim 9 further comprising a stepof annealing the product.